A SYSTEM OF RADIO-LOCATION USED IN THE DELTA AREA

R.H.J Morra

Abstract

In the South-Western part of the Netherlands the Delta project is being carried out consisting inter alia of k main dams closing k large inlets (fig.1). Through the four tidal inlets to be closed about 1800 million cubic metres of water run into the Delta area during flood tide and flow out again during ebb tide. This means about 7000 million cubic metres daily. The bottom of the inlets and the sea-bottom consist of fine sand (deQ = .100 - .2500 mm), which is in constant movement. During the past centuries considerable changes in the bottom contour have taken place. Yearly many millions of cubic metres of sand are moved by the water. The bottom is a very complicated system of gullies and sandbanks which has evolved down the centuries and is ever changing. It is evident that the dams under construction will out off the tidal flow into and out of the area and that this will result in a considerable change in the sand movement. The underwater estuary extending as far as 20-25 km seawards from the dams will probably have to adapt itself rather suddenly, i.e., within a few decennia. This can be dangerous for the Western extremities of the islands. The whole combination of phenomena concerned has to be studied and watched very carefully. Basic information concerning the sand movement and its consequences is given by soundings. A long term of frequent and accurate soundings with very good repeatability i3 required for the entire coastal area of the estuary. However, the meteorological conditions for sounding are such that good conditions only obtain on about 20 days a year, because good visibility and a quiet sea must occur simultaneously. Moreover, there is a serious shortage of landmarks and at distances seawards of less than 10 km from the shore visual location is impossible. The only solution is to make the location independent of visibility. For these reasons it was decided that a system of radio-location should be devised. We were advised by an independent expert to adopt the Decca survey system. With such a system it is possible to make frequent soundings with very good repeatability and with a reasonably low number of launches, because many more suitable days (and nights) become available. This system of radio-location is called the Delta chain. The system is also used for special purposes (velocity and sand transport measurements).